Lets say purely for example my site is a graphic design marketplace and I post my site on graphic design forums then the graphic designers on the forum will be able to access my site, register and apply for graphic design jobs.

Your links within the forums will likely be nofollow. Google will ignore your posts/links.

If you are posting on relevant forums, you may get some traffic from interested parties dependent upon your posting skills and the reputation you build on the forum(s). If the forum admins/moderators believe you are spamming their forum(s), they will likely penalize/ban you and delete your posts. If you are a true contributor, you may get some traffic/customers from those relevant forums.